Templated metal particles and methods of making

ABSTRACT

Composite particles of a metal particle within a crosslinked, cored dendrimer are described. Additionally, methods of making the composite particles and compositions that contain the composite particles are described.

RELATED APPLICATIONS

This application is a divisional application of Ser. No. 11/094,999filed on Mar. 31, 2005, now U.S. Pat. No. 7,344,583, the disclosure ofwhich is incorporated herein by reference.

BACKGROUND

Metal particles in the nanometer size range (e.g., 1 to 100 nanometersor 1 to 10 nanometers) are of increasing interest in a variety ofapplications such as catalysis, electronic devices, and sensors. Many ofthe methods used to prepare metal particles in this size range, however,are problematic.

Particles such as noble metal particles in the nanometer size range havebeen prepared by reacting soluble metal-containing precursors with areducing agent such as sodium borohydride in the presence of analkylthiol capping agent. While this method is effective for producingparticles in the nanometer size range, the particle surfaces are usuallypassivated (i.e., surface functionalized). A passivated noble metaltends to be less effective than non-passivated noble metals inapplications such as catalysis or sensing.

Metal particles in the nanometer size range have been prepared withindendritic structures. The dendritic structures have been loaded withcations from a soluble metal salt or acid and then reduced to producemetal particles. These metals are typically coordinated on the outersurface by functional groups within the dendritic structure. Suchcoordination can disadvantageously affect the utility of the metalparticles for various applications such as catalysis or sensing.

SUMMARY OF INVENTION

Metal particles and a method of making metal particles in the nanometersize range are disclosed. More specifically, the metal particles areformed within a crosslinked dendritic structure that has the coreorganic material (e.g., the zero generation material) or the coreorganic material plus other lower generation material removed.

In a first aspect, a method of making metal particles is provided. Themethod involves providing a dendrimer that contains a core organicmaterial, a first dendron bonded to the core organic material through afirst attachment group that can be chemically cleaved, and a seconddendron bonded to the core organic material through a second attachmentgroup that can be chemically cleaved. The first dendron has at least twofirst crosslinkable groups and the second dendron has at least twosecond crosslinkable groups. The method further involves forming acrosslinked dendrimer by reacting the first crosslinkable groups and thesecond crosslinkable groups; cleaving by a chemical reaction both thefirst attachment group and the second attachment group; removing thecore organic material or a derivative thereof from the crosslinkeddendrimer to form a cored dendrimer having interior end groups; forminga coordinative bond or an ionic bond between a metal-containingprecursor and at least one interior end group within a central region ofthe cored dendrimer; and reducing the metal-containing precursor to forma metal particle within the central region of the cored dendrimer.

In a second aspect, a composite particle is provided that includes ametal particle within a central interior region of a cored dendrimer.The cored dendrimer has crosslinked dendrons surrounding the centralinterior region and the central interior region is free of organicmaterial. The metal particle has a size that is no greater than an outerdimension of the cored dendrimer.

In a third aspect, a composition is provided that includes an organicmatrix and a composite particle in the organic matrix. The compositeparticle includes a metal particle within a central interior region of acored dendrimer. The cored dendrimer has crosslinked dendronssurrounding the central interior region and the central interior regionis free of organic material. The metal particle has a size that is nogreater than an outer dimension of the cored dendrimer.

As used herein, the term “dendron” refers to a molecular structurehaving a plurality of molecular branching groups that divide a singlemolecular chain into two or more molecular chains. Additional branchinggroups can further divide a previously divided molecular chain. Themolecular chains can be aromatic, aliphatic, heterocyclic, or acombination thereof.

As used herein, the term “dendrimer” or “dendritic structure” refers toa molecular structure that includes at least two dendrons attached to acentral molecular species (i.e., the core organic material).

As used herein, the term “crosslinked dendrimer” refers to a dendrimerin which at least two molecular chains of one dendron are crosslinked tomolecular chains of one or more other dendrons. Optionally, a molecularchain within one dendron can be crosslinked to another molecular chainwithin the same dendron. The crosslinks can be along the length of amolecular chain or can be at the outer periphery of a molecular chain ofa dendron.

As used herein, the term “cored dendrimer” refers to a crosslinkeddendrimer that has been chemically reacted to remove an interior region.That is, the core organic material (i.e., central molecular species) orderivative thereof has been removed from the crosslinked dendrimer toform the cored dendrimer. The cored dendrimer can resemble a crosslinkedpolymeric sphere that is partially hollow (i.e., the central region ofthe cored dendrimer is hollow and free of organic material).

As used herein, the term “coordinating” or “coordination” refers toforming a covalent bond between two species where one of the speciescontributes both electrons. Such a bond is sometimes referred to as adative bond, as a “coordinative covalent bond”, as a “coordinativebond”, or as “coordinated”.

As used herein, the term “acyl” refers to a monovalent group of formula—(CO)R^(a) where R^(a) is an alkyl, aryl, or heterocycle group and where(CO) used herein indicates that the carbon is attached to the oxygenwith a double bond.

As used herein, the term “acyloxy” refers to a monovalent group offormula —O(CO)R^(a) where R^(a) is an alkyl, aryl, or heterocycle group.

As used herein, the term “acyloxycarbonyl” refers to a monovalent groupof formula —(CO)O(CO)R^(a) where R^(a) is an alkyl, aryl, or heterocyclegroup.

As used herein, the term “alkyl” refers to a monovalent group formedfrom an alkane and includes groups that are linear, branched, cyclic, orcombinations thereof. The alkyl group typically has 1 to 30 carbonatoms. In some embodiments, the alkyl group contains 1 to 20 carbonatoms, 1 to 10 carbon atoms, 1 to 6 carbon atoms, or 1 to 4 carbonatoms. Examples of alkyl groups include, but are not limited to, methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl,n-hexyl, cyclohexyl, n-heptyl, n-octyl, and ethylhexyl. The alkyl groupcan be unsubstituted or substituted with one or more substituentsselected from, for example, hydroxy, alkoxy, amino, aryl, or halogroups.

As used herein, the term “alkoxy” refers to a monovalent group offormula —OR^(b) where R^(b) is an alkyl.

As used herein, the term “amino” refers to monovalent group of formula—NHR^(c) where R^(c) is hydrogen, alkyl, aryl, or heterocycle.

As used herein, the term “aryl” refers to a monovalent aromaticcarbocyclic radical. The aryl can have one aromatic ring or can includeup to 5 carbocyclic ring structures that are connected to or fused tothe aromatic ring. The other ring structures can be aromatic,non-aromatic, or combinations thereof. Examples of aryl groups include,but are not limited to, phenyl, biphenyl, terphenyl, anthryl, naphthyl,acenaphthyl, anthraquinonyl, phenanthryl, anthracenyl, pyrenyl,perylenyl, and fluorenyl. An aryl group can be unsubstituted orsubstituted, for example, with one or more substituents selected, forexample, from alkyl, alkoxy, halo, hydroxy, amino, or combinationsthereof.

As used herein, the term “borono” refers to a group of formula —B(OH)₂.

As used herein, the term “carbonyl” refers to a divalent group offormula —(CO)— where there is a double bond between the carbon andoxygen.

As used herein, the term “carboxy” refers to a monovalent group offormula —(CO)OH.

As used herein, the term “chlorophosphono” refers to a monovalent groupof formula —OP(O)(OR^(c))Cl where R^(c) is hydrogen, alkyl, aryl, orheterocycle.

As used herein, the term “chlorophosphito” refers to a monovalent groupof formula —OP(OR^(c))Cl where R^(c) is hydrogen, alkyl, aryl, orheterocycle.

As used herein, the term “chlorosulfonyl” refers to a monovalent groupof formula —SO₂Cl.

As used herein, the term “chlorosilyl” refers to a monovalent group offormula —SiR^(d) ₂Cl where each R^(d) is independently an alkyl, aryl,or alkoxy.

As used herein, the term “formyl” refers to a monovalent group offormula —(CO)H.

As used herein, the term “halo” refers to a monovalent group of formula—F, —Cl, —Br, or —I.

As used herein, the term “halocarbonyl” refers to a monovalent group offormula —(CO)X where X is a halo. A chlorocarbonyl is one example of ahalocarbonyl group.

As used herein, the term “heterocycle” or “heterocyclic” refers to acyclic group or cyclic compound that is aromatic or non-aromatic andthat contains at least one heteroatom selected from O, N, S, P, or Si.The heterocyclic group or compound can include one ring or can containup to 5 rings, up to 4 rings, up to 3 rings, or 2 rings that are fusedor connected where at least one ring contains a heteroatom. Exemplaryheterocyclic groups or compounds contain up to 20 carbon atoms, up to 15carbon atoms, up to 10 carbon atoms, or up to 5 carbon atoms and up to 5heteroatoms, up to 4 heteroatoms, up to 3 heteroatoms, or up to 2heteroatoms. Some heterocyclic groups are five-membered rings orsix-membered rings with 1, 2, or 3 heteroatoms.

As used herein, the term “hydroxy” refers to a monovalent group offormula —OH.

As used herein, the term “isocyanto” refers to a monovalent group offormula —NCO.

As used herein, the term “mercapto” refers to a monovalent group offormula —SH.

As used herein, the term “phosphono” refers to a monovalent group offormula —P(O)(OH)(OR^(c)) where R^(c) is hydrogen, alkyl, aryl, orheterocylic.

As used herein, the term “phosphate” refers to a monovalent group offormula —OP(O)(OH)(OR^(c)) where R^(c) is hydrogen, alkyl, aryl, orheterocyclic.

As used herein, the term “phosphonamino” refers to a monovalent group offormula —NHP(O)(OH)(OR^(c)) where R^(c) is hydrogen, alkyl, aryl, orheterocyclic.

As used herein, the term “silanol” refers to a monovalent group offormula —SiR^(d) ₂(OH) where each R^(d) is independently alkyl, aryl, oralkoxy.

As used herein, the term “sulfamino” refers to a monovalent group offormula —NHS(O)₂(OH).

As used herein the term “sulfono” refers to monovalent group of formula—S(O)₂(OH).

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures, Detailed Description, and Examples that followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a transmission electron micrograph of a composite particlethat contains silver; and

FIG. 2 shows an illustrative particle size distribution for a compositeparticle that contains silver.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

Composite particles that include metal particles within a central regionof a cored dendrimer are described. The cored dendrimer can function asa template for the formation of metal particles and typically definesthe maximum size of the metal particles.

In one aspect, a method of making metal particles is disclosed. Adendrimer is provided that includes a core organic material, a firstdendron bonded to the core organic material through a first attachmentgroup that can be chemically cleaved, and a second dendron bonded to thecore organic material through a second attachment group that can bechemically cleaved. Each dendron has at least two crosslinkable groups(the first dendron has at least two first crosslinkable group and thesecond dendron has at least two second crosslinkable groups). At leasttwo of the first crosslinkable groups and at least two of the secondcrosslinkable groups are reacted to form a crosslinked dendrimer. Afterformation of the crosslinked dendrimer, the first attachment group andthe second attachment group are cleaved by a chemical reaction. The coreorganic material or a derivative of the core organic material is removedfrom the crosslinked dendrimer to form a cored dendrimer having interiorend groups. A coordinative bond or ionic bond is formed by reacting ametal-containing precursor with at least one of the interior end groupswithin a central region of the cored dendrimer. The metal-containingprecursor is then reduced to form a metal particle within the centralinterior region of the cored dendrimer.

One embodiment of the method is shown schematically in Reaction SchemeA. In this reaction scheme, three dendrons are bonded to the centralcore organic material through attachment groups. The bonding of thedendrons to the core organic material results in the formation of adendrimer (i.e., Structure I). The attachment groups (shown bycomplementary structures for a triangle, square, and circle in StructureI) can be the same or different (e.g., as shown, the attachment groupsare different). The X groups on each dendron represent groups that arecrosslinkable (e.g., as shown, each dendron has two crosslinkablegroups). The E groups represent the peripheral groups (i.e., the endgroups furthest from the core organic material) at the outer peripheryof the branched molecular chains in the dendrons. Structure IIschematically represents the crosslinked dendrimer (i.e., the C—C bondis used to represent the crosslinks formed between two dendrons).Structure III schematically represents the cored dendrimer (i.e., thecored dendrimer has the core organic material removed from a crosslinkeddendrimer and has internal end groups that can be bonded to ametal-containing precursor). The internal end groups within the coreddendrimer can be the same or different than the groups on the dendronsoriginally reacted with the core organic material. As shown, the circle,triangle, and square in Structure III represent the end groups withinthe interior of the cored dendrimer. Structure IV schematicallyrepresents a metal particle within the interior of the cored dendrimer.The size of the metal particle is no greater than an outer dimension ofthe cored dendrimer. The outer surface of the metal particle is notbonded to the cored dendrimer (i.e., the outer surface of the metalparticle is free of coordinative bonds or ionic bonds to the coreddendrimer).

The core organic material (i.e., the central organic material of adendrimer) can be aliphatic, aromatic, heterocyclic, or a combinationthereof and has at least two reactive groups that can form a covalentbond by reacting with a functional group on a dendron. Suitable coreorganic materials often contain up to 100 carbon atoms, up to 90 carbonatoms, up to 80 carbon atoms, up to 70 carbon atoms, up to 60 carbonatoms, up to 50 carbon atoms, up to 40 carbon atoms, up to 30 carbonatoms, up to 20 carbon atoms, or up to 10 carbon atoms. Some coreorganic materials contain at least three, at least four, at least five,at least six, at least seven, or at least eight reactive groups. Eachreactive group of the core organic material can be a bonding site for adendron. Exemplary reactive groups include, but are not limited to,hydroxy, halo, carboxy, halocarbonyl, amino, mercapto, sulfono,sulfamino, phosphono, phosphonamino, acyl, borono, silanol, chlorosilyl,trifluoromethylsulfonatesilyl, chlorosulfonyl, chlorophosphono,chlorophosphito, and isocyanto.

In some embodiments, the core organic material includes an aromaticcompound having at least three reactive groups. The aromatic compoundcan be carbocyclic or heterocyclic and can have multiple rings.Exemplary carbocyclic aromatic compounds include substituted benzenecompounds such as 1,3,5-tris-hydroxy-benzene;1,3,5-tris-hydroxymethyl-benzene; 1,3,5-tris-bromomethyl-benzene;1,3,5-benzenetricarboxylic acid chloride; and the like. Exemplarycarbocyclic aromatic compounds having multiple rings include, but arenot limited to, 1,4-bis[3,5-dihydroxybenzoyloxy]benzene;1,1,1-tris(4-hydroxyphenyl)ethane; 1,1,1-tris{4-[4,4-bis(4-hydroxyphenyl)pentoxy]phenyl}ethane; and cyclophanes.Exemplary heterocyclic aromatic compounds include substituted porphinecompounds such as 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphine,5,10,15,20-tetrakis(2,6-dihydroxyphenyl)-21H,23H-porphine, and5,10,15,20-tetrakis(3,5-dihydroxyphenyl)-21H,23H-porphine.

In other embodiments, the core organic material includes an aliphaticcompound having at least three reactive groups. Exemplary aliphaticcompounds include substituted alkanes such as2-ethyl-2-hydroxymethyl-propane-1,3-diol;2,2-bis-hydroxymethyl-propane-1,3-diol (i.e., pentaerythritol);1,3-dibromo-2,2-bis-bromomethyl-propane;3-[3-(2-chlorocarbonyl-ethoxy)-2,2-bis-(2-chlorocarbonyl-ethyoxymethyl)-propoxy]-propionylchloride; pentaerythritol ethoxylate (i.e., C[CH₂(OCH₂CH₂)_(n)OH]₄ wheren is an integer of 1 to 10); pentaerythritol propoxylate (i.e.,C[CH₂[OCH₂CH(CH₃)]_(n)OH]₄ where n is an integer of 1 to 10); and thelike.

The dendrimer can be formed using a divergent or convergent methodology.In a divergent methodology, the dendritic structure is typicallyconstructed through sequential addition of monomers beginning with thecore organic material and radiating outward. The core organic materialcan be referred to as the zero generation. Adding enough monomer toreact with all of the reactive sites on the core organic materialresults in the formation of the first generation dendritic structure. Amonomer with at least two additional reaction sites can function as abranching group that splits a single molecular chain into two or threemolecular chains. Repetitive addition of more monomer results in theformation of second, third, fourth, fifth, and higher generationdendritic structures. In a convergent methodology, the dendrons aresynthesized and then covalently bonded to the core organic material.

The dendrons can be aromatic, aliphatic, or combinations thereof and canhave a plurality of branching groups. The dendrons can includeheteroatoms. Some exemplary dendrons are poly(ethers), poly(esters),poly(thioethers), poly(arylalkylene ethers), poly(amidoamines),poly(alkylene imine), and the like. The reference by G. R. Newkome etal., Dendrimers and Dendrons: Concepts, Synthesis, Applications,Wiley-VCH, New York, pp. 1-623 (2001) describes these and other suitabledendrons.

In some embodiments, the dendrons are poly(ethers) such as, for example,poly(benzyl ethers). For example, the dendrons can have a structure suchas

or higher generation analogues thereof. In this exemplary thirdgeneration dendron, the crosslinkable groups are alkenyl groups and thelocal functional group that can react with the core organic material isa carboxy group. The benzene ring with the carboxy group is the firstgeneration region of the dendron. This dendron and other suitableexemplary dendrons are described further in S. C. Zimmerman et al.,Nature, vol. 418, 399-403 (2002) and Wendland et al., J. Am. Chem. Soc.,121, 1389-1390 (1999).

Each dendron has a focal functional group that can be combined with areactive group of the core organic material to form an attachment grouplinking the dendron to the core organic material. The bonding of two ormore dendrons to the core organic material results in the formation of adendrimer. The attachment group is typically a group that contains achemical bond that can be cleaved (i.e., reacted) to provide an endgroup within the interior of the cored dendron that is capable ofbonding with a metal-containing precursor (e.g., capable of forming acoordinative bond or an ionic bond with a metal-containing precursor).Suitable attachment groups are those that can be attached to the coreorganic material such that the resulting chemical bond can be cleaved toform an end group such as, for example, a carboxy, hydroxy, amino,mercapto, sulfono, sulfamino, phosphono, phosphonamino, phosphate,borono, silanol, formyl, or acyl within the interior of the coreddendrimer.

The attachment group links the dendrons to the core organic material.Each linkage can be represented by the formulaDen-A-Corewhere Den represents the dendron, A represents the attachment group, andCore represents the core organic material. Each core has at least twosuch linkages (i.e., each dendrimer has at least two dendrons attachedto the core organic material). For ease of discussion, however, theformula shows only one of the two or more linkages to the core organicmaterial.

The linkage denoted by Den-A-Core can be, for example, an anhydride offormula

a mixed anhydride formed from a carboxylic acid and sulfonic acid offormula

a carboxylate ester of formula

a thioester of formula

a dithioester of formula

a succinimide ester of formula

an amide of formula

a thioamide of formula

a carbonate of formula

a dithiocarbonate-O-ester of formula

a carbamate of formula

a thiocarbamate-O-ester of formula

a thiocarbamate-S-ester of formula

a dithiocarbamate of formula

a sulfonate of formula

a sulfamide of formula

a sulfonamide of formula

a phosphonate of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a thiophosphonate of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a phosphoramidate of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a thiophosphonamide of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a phosphoramidite of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a phosphate of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a siloxane of formula

where each R^(d) is independently an alkyl, alkoxy, or aryl;a silazane of formula

where each R^(d) is independently an alkyl, alkoxy, or aryl;a peroxide of formula

a disulfide of formula

a boronic ester of formula

an acetal of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a di-substituted alkene of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;an imine of formula

where R^(c) is hydrogen, alkyl, aryl, or heterocyclic;a photolytically labile ester such as, for example,

where R^(b) is an alkyl;or photolytically labile carbamates such as, for example,

where R^(b) is an alkyl.

As used herein, multiple structures for a type of Den-A-Core aredesignated with a number such as a (1) or (2). For example, using thedesignation numbers in the list above, carboxylate ester (1) refers tothe structure

and carboxylate ester (2) refers to the structure

As used herein, the recitation of two or more cores in a formula for thelinkage Den-A-Core means that the dendron is covalently attached to thecore at multiple locations. For example, a boronic ester, an acetal,some photolytically labile esters, and some photolytically labilecarbamates are bonded to the core organic material in at least twolocations.

The Den-A-Core linkages can be prepared by any known method. Some ofthese linkages can be formed by reaction of an electrophilic group(e.g., an activated ester) with a nucleophilic group such as a hydroxygroup of an alcohol, an amino group of an amine, or a mercapto group ofa mercaptan. The nucleophilic group can be either on the core organicmaterial or on the dendron.

In some embodiments, Den-A-Core is a carboxylate ester. Various chemicalapproaches can be used to prepare such a linkage. For example, a dendronhaving a carboxy group (i.e., the dendron is a carboxylic acid) can bereacted with a hydroxy group on the core organic material (i.e., thecore organic material is an alcohol) to form carboxylate ester (1).

This reaction can occur in the presence of one or more reagents such as,for example, DCC and either DMAP or pyridine; DCC and DPTS; EDC andeither DMAP or pyridine; DCC and HBT; DCC and PPY; or PPh₃ and eitherDEAD or DIAD. As used herein, the term “DCC” refers toN,N′-dicyclohexylcarbodiimide. As used herein, the term “DMAP” refers to4-(dimethylamino)pyridine. As used herein, the term “DPTS” refers to a1:1 combination of DMAP and p-toluene sulfonic acid. As used herein, theterm “EDC” refers to N-(3-dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride. As used herein, the term “HBT” refers to1-hydroxy-1H-benzotriazole. As used herein, the term “PPY” refers to4-(pyrrolidin-1-yl)pyridine. As used herein, the term “PPh₃” refers totriphenylphosphine. As used herein, the term “DEAD” refers to diethylazodicarboxylate. As used herein, the term “DIAD” refers to diisopropylazodicarboxylate.

Alternatively, a carboxylate ester (1) linkage can be formed by reactingthe carboxy group on a dendron with thionyl chloride (SOCl₂) or oxalylchloride (COCl)₂ in the presence of a base catalyst to form a dendronhaving a chlorocarbonyl group. The chlorocarbonyl group then can bereacted with a hydroxy group on the core organic material (i.e., thecore organic material is an alcohol) in the presence of a reagent suchas pyridine or DMAP.

In other embodiments, Den-A-core is carboxylate ester (2). As withcarboxylate ester (1), various chemical approaches can be used toprepare such a linkage. For example, a core organic material having acarboxy group (i.e., the core organic material is a carboxylic acid) canbe reacted with a dendron having a hydroxy group (i.e., the dendron isan alcohol).

This reaction can be conducted in the presence of the same reagents thatare suitable for preparing carboxylate ester (1). Alternatively, acarboxy group on the core organic material can be reacted with thionylchloride or oxalyl chloride in the presence of a base catalyst to form acore organic material having a chlorocarbonyl group. The chlorocarbonylgroup can be reacted with a hydroxy group on the dendron (i.e., thedendron is an alcohol) in the presence of a weak base such as pyridineor DMAP.

Amide linkages can be prepared using reactions similar to those used toprepare carboxylate esters. However, in the above reactions, a primaryor secondary amine would be used in place of an alcohol as thenucleophile. More specifically, a dendron having a carboxy group (i.e.,the dendron is a carboxylic acid) can be reacted with a core organicmaterial having an amino group (i.e., the core organic material is anamine) to form amide (2).

Alternatively, a dendron having an amino group (i.e., the dendron is anamine) can be reacted with a core organic material having a carboxygroup (i.e., the core organic material is a carboxylic acid) to formamide (1).

As with the carboxylate esters, the carboxy groups can be converted tochlorocarbonyl groups prior to reaction with an amine.

Thioester linkages can be prepared using similar reactions to those usedto form carboxylate esters and amides. However, a mercaptan is used inplace of an alcohol or amine as the nucleophile. More specifically, adendron having a carboxy group (i.e., the dendron is a carboxylic acid)can be reacted with a core organic material having a mercapto group(i.e., the core organic material is a mercaptan) to form thioester (1).

Alternatively, a dendron having a mercapto group (i.e., the dendron is amercaptan) can be reacted with a core organic material having a carboxygroup (i.e., the core organic material is a carboxylic acid) to formthioester (2).

In yet other embodiments, the Den-A-Core linkage is an anhydride. Such alinkage can be formed by initially reacting a carboxy group on a dendronwith thionyl chloride or oxalyl chloride in the presence of a basecatalyst to form a dendron having a chlorocarbonyl group. Thechlorocarbonyl group then can be reacted with a carboxy group on thecore organic material in the presence of a weak base such as DMAP orpyridine.

Alternatively, a carboxy group on the core organic material can bereacted with thionyl chloride or oxalyl chloride in the presence of abase catalyst to form a core organic material having a chlorocarbonylgroup. The chlorocarbonyl group can be reacted with a carboxy group onthe dendron to form an anhydride linkage.

Some other Den-A-Core linkages are siloxanes. Reacting a chlorosilylgroup of a chlorosilane core organic material with a dendron that is analcohol can form siloxane (2). The reaction is usually conducted in thepresence of a base (e.g., triethylamine, imidazole, sodium hydroxide, orthe like). Each R^(d) group of the chlorosilyl group can beindependently selected from an alkyl, aryl, or alkoxy group.

Alternatively, reacting a trifluoromethylsulfonatesilyl group of atrifluormethylsulfonate silane core organic material with a dendronhaving a hydroxy group can form a siloxane (2) linkage. The reaction isusually conducted in the presence of a base. Each R^(d) group of thetrifluoromethylsulfonatesilyl group can be independently selected froman alkyl, aryl, or alkoxy group.

Similarly, reacting a chlorosilane or a trifluoromethylsulfonate silanedendron with a core organic material that is an alcohol can formsiloxane (1).

Carbamate linkages can be formed, for example, by reacting an alcoholwith an isocyanate. The isocyanato group can be on either the coreorganic material or the dendron as shown in the following reactions.

Thiocarbamate-S-ester linkages can be formed using similar reactions tothose used to form carbamate linkages. Mercapto groups rather thanhydroxy groups on the core or dendron can be reacted with an isocyanatogroup.

Either the core organic material or the dendron can be a mercaptan.These thiocarbamate-S-esters can be easier to cleave than carbamates andthiocarbamate-O-esters.

Sulfonamide linkages can be formed by reacting an amino group with achlorosulfonyl group in the presence of a mild base such as DMAP orpyridine. Either the core organic material or the dendron can be anamine.

Phosphoramidate linkages can be formed by first reacting an alkyldichlorophosphate (e.g., ethyldichlorophosphate) with an alcohol in thepresence of a mild base such as pyridine or DMAP to form achlorophosphono group. The chlorophosphono group can then be reactedwith an amine. In some examples, the core organic material is an alcoholand the dendron is an amine.

In other examples, the dendron is an alcohol and the amino group is onthe core organic material.

Phosphoramidite linkages can be formed by first reacting an alkyldichlorophosphite (e.g., ethyldichlorophosphite) to produce achlorophosphito group in the presence of a mild base such as pyridine orDMAP. The chlorophosphito group can be reacted with an amino group. Insome examples, the amino group is on the dendron (i.e., the dendron isan amine and the core organic material is an alcohol).

Alternatively, the amino group can be on the core organic material andthe dendron is an alcohol that reacts with the alkyldichlorophosphite.

Dithiocarbonate (i.e., xanthate) linkages can be formed by reacting ahalo group with a salt of a dithiocarbonic acid-O-ester. In someexamples, the halo group is on the dendron.

In other examples, the halo group is on the core organic material.

A phosphonate linkage can be formed by reacting a hydroxy group and aphosphono group using Mitsunobu esterification conditions (e.g., in thepresence of PPh₃ and either DEAD or DIAD). In some examples, the hydroxygroup is on the dendron (i.e., the dendron is an alcohol and the coreorganic material is a phosphonate).

In other examples, the hydroxy group is on the core organic material(i.e., the core organic material is an alcohol and the dendron is aphosphonate).

Boronic ester linkages can be formed by reacting a boronic acid with acore organic material having at least two hydroxy groups. The boronicester includes two covalent bonds to the core organic material. That is,the boronic ester is attached to the core in two places.

After attachment of the dendron to the core organic material, thedendrons are crosslinked to form a crosslinked dendrimer. Each dendronhas at least two crosslinkable groups that can be used to crosslink onedendron to at least one other dendron. Each dendron is crosslinked toone or two other dendrons. When the dendrimer is prepared from twodendrons, there are two crosslinks between the first dendron and thesecond dendron. When the dendrimer is prepared from three or moredendrons, a first dendron can be crosslinked to a second dendron and toa third dendron. That is, when there are three or more dendrons in adendrimer, each dendron is crosslinked to two other dendrons.

A first dendron has at least two first crosslinkable groups and a seconddendron has at least two second crosslinkable groups. To form acrosslinked dendrimer, the first crosslinkable groups and the secondcrosslinkable groups are reacted. The first and second crosslinkablegroups can be reacted with each other or with other crosslinkablegroups. In some examples, such as in dendrimers that are formed fromonly two dendrons, two of the first crosslinkable groups are crosslinkedwith two of the second crosslinkable groups. In some other examples,such as in dendrimers that are formed from at least three dendrons, oneof the first crosslinkable groups on the first dendron is crosslinkedwith one of the second crosslinkable groups on the second dendron, oneof the first crosslinkable groups on the first dendron is reacted with acrosslinkable group on the third dendron, and one of the secondcrosslinkable groups on the second dendron is reacted with acrosslinkable on a third dendron. This example is shown in ReactionScheme A.

Additionally, a molecular chain of one dendron can be crosslinked toanother molecular chain of the same dendron. Some dendrons have two,three, four, five, six, seven, eight, nine, ten, or more than tencrosslinkable groups. Within a dendron, the crosslinkable group can belocated along the length of a molecular chain or at an outermostperiphery of a molecular chain (e.g., as shown in Reaction Scheme A). Insome embodiments, a dendron has a crosslinkable group at the outermostperiphery of each outer molecular chain (i.e., at the end of each outerbranch of the dendron as shown in Reaction Scheme A).

The crosslinkable groups can be reacted to form reversible orirreversible crosslinks. The crosslink density is sufficiently high toprevent the disassembly of the dendritic structure upon removal of thecore organic material. That is, the crosslinks allow the crosslinkeddendrimer to remain intact even after removal of the central coreorganic material in the process of forming a cored dendrimer. On theother hand, the crosslink density is sufficiently low to allowsubsequent removal of the core organic material without disrupting thecrosslinks. Additionally, the crosslink density is sufficiently low toallow a metal-containing precursor and other reagents to enter into thecored dendrimer.

In one method of forming the crosslinks, a first crosslinkable groupreacts directly with a second crosslinkable group. In another method offorming the crosslinks, a difunctional crosslinking compound is reactedwith both the first crosslinkable group and the second crosslinkablegroup.

In the following formulas, Den₁ denotes the first dendron exclusive ofone of the first crosslinkable groups while Den₂ denotes the seconddendron exclusive of one of the second crosslinkable groups. Each Den₁and Den₂ is also attached to the core organic material. For ease ofdescribing the crosslinking reactions, the core organic material is notincluded in the reactions. Each Den₁ and Den₂ includes at least oneother crosslinkable group. For ease of describing the crosslinkingreactions, only one crosslinkable group is included in the reactions.

Various chemical approaches can be used to form a crosslink by directlyreacting a first crosslinkable group with a second crosslinkable group.In a first example, a metathesis catalyst can be added to crosslink twoalkene groups as shown in the following reaction.

Exemplary metathesis catalysts include, but are not limited to,ruthenium, molybdenum, tungsten, or rhenium complexes. The alkenes areoften monofunctional. Replacing a hydrogen atom on the alkenyl grouptends to decrease the rate of the crosslinking reaction.

Another example of directly reacting the crosslinkable groups involves a[2+2]cycloaddition reaction in the presence of light.

The reactants can be stilbenes (e.g., Z₁ and Z₂ are phenyl), cinnamates(e.g., Z₁ and Z₂ are carboxy or carboxylate groups), or uracils. In amore specific example, the reactants are uracils such as describedfurther in Tominaga et al., Chemistry Letters, 374-375 (2000).

Additionally, the crosslinkable groups can be directly reacted usingtypical polymerization reactions. For example, acrylates, methacrylates,acrylamides, and methacrylamides can be crosslinked in the presence of athermal of photo initiator.

In this reaction, z₁ and z₂ are each independently hydrogen or methyl;Y₁ and Y₂ are each independently oxygen or NR^(c) where R^(c) ishydrogen, alkyl, aryl, or heterocyclic; and n is an integer of 1 to 100.In other polymerization crosslinking reactions, vinyl ethers or vinylesters can undergo a cationic polymerization reaction as shown belowwhere n is an integer of 1 to 100.

In yet other polymerization crosslinking reactions, epoxides can undergoring-opening polymerization reactions in the presence of an acid oramine catalyst where n is an integer of 1 to 100.

Another method of crosslinking dendrons involves the formation of acoordination crosslink with a metal species. Both the first and secondcrosslinkable groups ligate to a single metal species. For example,phenanthrenyl groups can form a coordination crosslink in the presenceof a metal salt (M) such as a copper salt.

As another example of a direct reaction, each crosslinkable group can bea cyclobutylphenyl group. Upon heating, two such groups can form atricyclic structure as depicted in the following reaction.

In the above crosslinking reactions, a first dendron is crosslinked to asecond dendron. The first dendron and the second dendron are often partof the same dendritic structure (i.e., the dendrons that are crosslinkedare attached to the same core organic material). However, in someexamples, the dendrons that are crosslinked are in different dendriticstructures (i.e., the dendrons that are crosslinked are not attached tothe same core organic material). When it is desired that thecrosslinking is between dendrons attached to the same core organicmaterial, the crosslinking reactions often occur in a medium with arelatively low amount of the dendrimer to minimize the interactionsbetween different dendritic structures.

In the second method of forming a crosslinked dendrimer, variousmultifunctional (e.g., difunctional) crosslinking agents can be used tojoin two dendrons. The crosslinking agent can react with both the firstcrosslinkable group and the second crosslinkable group. In someembodiments, the crosslinking agent is a nucleophile and thecrosslinkable groups are electrophiles. Alternatively, the crosslinkingagent is an electrophile and the crosslinkable groups are nucleophiles.Suitable crosslinking agents include difunctional activated carbonyls,difunctional azlactones, difunctional isocyanates, difunctional silanes,difunctional siloxanes, difunctional alkylhalides, difunctionalarylhalides, difunctional aldehydes, difunctional ketones, difunctionalaryl cupric halides, difunctional (meth)acrylates, difunctional(meth)acrylamides, difunctional alcohols, difunctional amines,difunctional compounds having hydroxy and amino groups, difunctionalcompounds having hydroxy and mercapto groups, difunctional compoundshaving mercapto and amino groups, difunctional alkylenes, difunctionalconjugated dienes, difunctional β-ketones, difunctional β-keto esters,difunctional β-keto amides, and the like. As used herein, the term“difunctional” refers to two functional groups of the typecharacteristic of the named compound. For example, a difunctional aminehas two amino groups and a difunctional isocyanate has two isocyanatogroups.

In some examples, the crosslinking agent is a difunctional carboxylicacid, or a difunctional, activated carbonyl such as a difunctionalcarboxylic acid halide, difunctional anhydride, or difunctionalsuccinimide ester. Such a crosslinking agent can be reacted with hydroxycrosslinkable groups, amino crosslinkable groups, or mercaptocrosslinkable groups on dendrons as exemplified by the followingreactions.

In these reactions, X is independently selected from chloro, bromo,hydroxy, acyloxy, or oxysuccinyl; and Q is a connecting group that isaliphatic, aromatic, heterocyclic, or a combination thereof. Althoughthe crosslinkable groups shown in these equations are the same, thecrosslinkable groups can be different (i.e., each crosslinkable groupcan be independently selected from an amino, hydroxy, and mercaptogroups). Alternatively, the crosslinking agent can have two functionalgroups independently selected from amino, mercapto, and hydroxy groupswith crosslinkable groups selected from halocarbonyl, carboxy,acyloxycarbonyl, or succinyloxycarbonyl groups.

In other examples, the crosslinking agent is a difunctional azlactone.Such a crosslinker could be reacted with amino crosslinkable groups orhydroxy crosslinkable groups as exemplified by the following reactions.

Each R^(c) group is independently hydrogen, alkyl, or aryl; and Q is aconnecting group that is aromatic, aliphatic, heterocyclic, or acombination thereof. Although the crosslinkable groups shown in theseequations are the same, one crosslinkable group can be a hydroxy and theother crosslinkable group can be an amino. Alternatively, the dendronscan be azlactones and the crosslinking agent can have two functionalgroups independently selected from an amino and hydroxy groups. In bothtypes of examples, the crosslinked reaction product is an ester oramide.

In still other examples, the crosslinking agent is a difunctionalisocyanate and the crosslinkable groups are amino groups or hydroxygroups.

In these reactions, Q is a connecting group that is aromatic, aliphatic,heterocyclic, or a combination thereof. Although the crosslinkablegroups shown in these equations are the same, one crosslinkable groupcan be a hydroxy and the other crosslinkable group can be an amino.Alternatively, the crosslinkable groups can be isocyanato groups and thecrosslinking agent can have two functional groups independently selectedfrom hydroxy and amino groups. In both types of examples, thecrosslinked reaction product is a urea or urethane.

A difunctional silyl-containing crosslinking agent can be reacted withalkenyl groups in the presence of a palladium catalyst.

In this reaction, each R^(d) is independently an alkyl, aryl, or alkoxygroup; and Q is a connecting group that is aromatic, aliphatic,heterocyclic, or a combination thereof. Alternatively, the crosslinkingagent can be a difunctional alkene (i.e., two carbon-carbon doublebonds) and the crosslinkable groups can be silyl groups of formula—Si(R^(d))₂H where R^(d) is the same as defined above. In both of theseexamples, the crosslinked reaction product is a silane.

A dihalogenated alkane crosslinking agent can be reacted with hydroxy,amino, or mercapto groups as exemplified in the following reaction wherethe crosslinkable group is a hydroxy.

In this reaction, each X is independently bromo, chloro, fluoro, oriodo; and Q is a connecting group that is aromatic, aliphatic,heterocyclic, or a combination thereof. Although both dendrons in thisreaction have the same crosslinkable group, each crosslinkable group canbe independently selected from hydroxy, amino, or mercapto groups. Incomplementary examples, the crosslinking agent has two functional groupsindependently selected from amino, mercapto, and hydroxy groups and eachcrosslinkable group is a haloalkyl.

A difunctional aldehyde or difunctional ketone crosslinking agent can bereacted with amino groups to form an imine as shown in the followingreaction where each R^(c) is independently hydrogen, alkyl, aryl, orheterocycle and Q is a connecting group that is aromatic, aliphatic,heterocyclic, or a combination thereof.

In complementary examples, the crosslinking agent is a difunctionalamine and the crosslinkable groups are independently selected fromformyl or acyl groups.

Aryl halide crosslinkable groups can react with a difunctional arylhalide that has been treated with copper to produce a difunctional arylcupric halide crosslinking agent using an Ullman reaction. In thisreaction, X is halo and Q is a connecting group that is aromatic,aliphatic, heterocyclic, or a combination thereof.

Crosslinks can also be formed using Michael addition reactions. In afirst example, the crosslinkable groups can be amino, hydroxy, ormercapto groups and the crosslinking agent can be a difunctional(meth)acrylate (i.e., difunctional acrylate or difunctionalmethacrylate) or difunctional (meth)acrylamide (i.e., difunctionalacrylamide or difunctional methacrylamide). Such a reaction is shown inthe following reaction for amino crosslinkable groups and a difunctionalacrylamide crosslinking agent.

In a second example, the crosslinkable group can be a β-diketone,β-diketo ester, or β-diketo amide and the crosslinking agent can be adifunctional (meth)acrylate or a difunctional (meth)arylamide. Such areaction is shown in the following reaction for a difunctionalacrylamide crosslinking agent and a β-diketone.

In these reactions, Q is a connecting group selected from an aromaticgroup, aliphatic group, heterocycle group, or a combination thereof.

A crosslink can be formed using a Diels-Alder reaction. That is, twocrosslinkable groups that are dienes can react with a crosslinking agentthat is a difunctional dienophile (i.e., there are two carbon-carbondouble bonds). An example is shown in the following reaction.

In this reaction, Q is a connecting group selected from an aromaticgroup, aliphatic group, heterocycle group, or a combination thereof.

After formation of the crosslinked dendrimer, the attachment groupslinking the dendrons to the core organic material are chemicallycleaved. The core organic material or a derivative of the core organicmaterial is removed from the crosslinked dendrimer. The structureremaining after removal of the core organic material or the derivativeof the core organic material is the cored dendrimer. The cored dendrimeris a crosslinked polymeric material (e.g., a crosslinked polymericsphere) that is at least partially hollow in the central, internalregion. The cored dendrimer has interior end groups that are capable offorming a coordinative bond or an ionic bond with a metal-containingprecursor such as, for example, a metal-containing salt (e.g., themetal-containing precursor contains a metal ion).

The core organic material can typically be freed from the crosslinkeddendrimer after cleavage of the attachment group. In some examples,however, the core organic material is further degraded such as throughhydrolysis or reduction reactions to allow more facile passage of thecore organic material from the interior of the crosslinked dendrimer.After removal of the core organic material from the crosslinkeddendrimer to form a cored dendrimer, additional regions of the dendronswithin the interior of the cored dendrimer can be removed and/or furthermodified such as through hydrolysis or reduction reactions. The removalof the core organic material corresponds to the removal of the zerogeneration of the dendrimer. Additional generations also can be removedsuch as, for example, the first generation or the first generation plusthe second generation from higher generation dendritic structures.

The cored dendrimer remains intact after removal of the core organicmaterial due, at least in part, to the existence of the crosslinks ofone dendron to another dendron, the existence of crosslinks between themolecular chains of a single dendron, or combinations thereof. Theorganic material removed from the crosslinked dendrimer that results inthe formation of a cored dendrimer typically does not involve theremoval of the crosslinks between dendrons or between molecular chainsof a dendron. At least one generation of the original dendrons used toform the dendritic structure remain in the cored dendrimer.

The lowest generation of the dendritic structure that remains afterremoval of the core organic material or a derivative thereof hasreactive groups that are referred to as the interior end groups of thecored dendrimer or end groups within the interior of the coreddendrimer. The end groups within the interior of the cored dendrimer areoften a carboxy, hydroxy, amino, mercapto, sulfono, sulfamino,phosphono, phosphonamino, phosphate, borono, silanol, formyl, or acylgroup. There are at least two end groups within the interior of thecored dendrimer and these end groups can be the same or different.

In some embodiments, cleavage of the attachment group and removal of thecore organic material or a derivative thereof results in the formationof a carboxy group as an end group of a cleaved dendron within theinterior of the cored dendrimer. The cleaved dendron refers to thegenerations of the dendron that remain after chemical cleavage of theattachment group. The cleaved dendron is of formula

where D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer.Carboxy end groups within the interior of a cored dendrimer can beformed under mild hydrolytic conditions from a Den-A-Core linkage suchas carboxylate ester (1), an anhydride, a succinimide ester, thioester(1), or mixed anhydride (1) of carboxylic acid and sulfonic acid. Thecarboxy end groups can also be formed under strong acid or strong baseconditions from amide (2). Additionally, a carboxy end group within theinterior of the cored dendrimer can be formed from photolytically labileester linkages by exposure to suitable actinic radiation.

In other embodiments, cleavage of the attachment group results in theformation of a hydroxy group as the end group of a cleaved dendronwithin the interior of the cored dendrimer. That is, the cleaved dendronis of formulaD—OHwhere D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer.Hydroxy end groups can be formed, for example, by exposing sulfonate(1), phosphonate (1), phosphate, carbonate, sulfamide (1),phosphoramidate (1), carboxylate ester (2), or siloxane (2) to mildhydrolysis conditions. Siloxane (2) can also be cleaved with fluoridesalts such as tetrabutyl ammonium fluoride. Hydrolyzing carbamate (1),dithiocarbamate-O-ester (2), or thiocarbamate-O-ester (2) in thepresence of a strong base can generate a hydroxy end group.Additionally, a peroxide can generate a hydroxy end group when exposedto heat or to a radical source.

In other embodiments, cleavage of the attachment group results in theformation of an amino group as the end group of the cleaved dendronwithin the interior of the cored dendrimer. That is, the cleaved dendronis of formulaD—NH₂where D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer.Amino end groups can be formed in the interior of the dendrimer byhydrolysis (e.g., under relatively strong conditions) or by reduction(e.g., using reagents such as NaBH₄, LiAlH₄, or the like) of amide (1),thioamide, carbamate (2), thiocarbamate-O-ester (1), dithiocarbamate(1), thiocarbamate-S-ester (1), sulfonamide (1), sulfamide (2), orsilazane (1). Phosphoramidite (1) and phosphoramidate (2) can be cleavedby alcohols or water under mild conditions. Additionally, amino endgroups can be formed by photolytically labile carbamates. Amino endgroups can also be formed by cleaving an ester with hydrazine.

In still other embodiments, cleavage of the attachment group can resultin the formation of a mercapto group as the end group of the cleaveddendron within the interior of the cored dendrimer. That is, the cleaveddendron can be of formulaD—SHwhere D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer.Mercapto end groups can be formed from, for example, thioesters (2),dithioesters, thiophosphonates, thiophosphonamides, and dithiocarbonates(1) by cleaving the attachment group under mild to strong hydrolysisconditions. Thiocarbamates-S-esters (2) and dithiocarbamates (2) can becleaved using strong hydrolysis conditions or mild reduction conditionssimilar to those used to generate amino end groups. Disulfides can becleaved using reduction conditions to provide mercapto end groups.

Additionally, cleavage of the attachment group can result in theformation of a sulfono group as the end group of the cleaved dendron inthe interior of the cored dendrimer. That is, the cleaved dendron is offormula

where D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer.These end groups can be formed from sulfonate (2) and mixed anhydride(2) under mild hydrolysis conditions. Mild hydrolysis conditions alsocan be used to form sulfamino end groups

from sulfamides (2).

Cleavage of the attachment group can result in the formation of aphosphono group as the end group of the cleaved dendron in the interiorof the cleaved dendrimer. That is, the cleaved dendron is of formula

where D represents the region of the dendron exclusive of the interiorend group that remains in the central region of the cored dendrimer andR^(c) is a hydrogen, alkyl, aryl, or heterocycle. The phosphono groupcan be formed from phosphonates (2) using mild hydrolysis conditions.Mild hydrolysis conditions can also be used to form phosphonamino endgroups

from phosphoramidates (2) or form phosphate end groups

from phosphormidites (2).

End groups within the interior of a cored dendrimer can be a borono

generated by hydrolysis of the corresponding boronic ester in water. Drepresents the region of the dendron exclusive of the interior end groupthat remains in the central region of the cored dendrimer.

Additionally, silanols

can be formed where D represents the region of the dendron exclusive ofthe interior end group that remains in the central region of the coreddendrimer and each R^(d) is independently an alkyl, aryl, or alkoxy.Silanols can be generated by hydrolysis of the corresponding siloxanes(1) in water or by cleavage with tetrabutylammonium fluoride.Additionally, silanols can be formed from silazanes (2) with a strongbase.

The end groups of dendrons within the interior of the cored dendrimercan be an acyl or formyl group

where R^(c) is hydrogen, alkyl, aryl, or heterocycle and D representsthe region of the dendron exclusive of the interior end group thatremains in the central region of the cored dendrimer. Variouschemistries can be used to generate an acyl or formyl end group. Forexample, imines can be hydrolyzed using mild conditions. Additionally,acetals can be cleaved with a dilute acid (e.g., dilute hydrochloricacid) in a solvent such as acetone. Further, a cis- ortrans-disubstituted alkene can be subjected to oxidative conditions(e.g., ozonolysis or various oxidizing agents such as sodium iodinate,etc.).

Post-cleavage modification reactions can be used to provide end groupsfor the dendrons within the cored dendrimer that are capable ofattracting a metal-containing precursor. For example, an activated estercan be treated with ammonia or ammonium hydroxide to generate an amide.The subsequently formed amide can be subjected to reduction conditions(e.g., NaBH₄) to form an amine.

In another example of a post-cleavage modification reaction, an estercan be hydrolyzed to an alcohol.

The subsequently formed alcohol can be reacted with an acid chloride(e.g., acetoacetic acid chloride) to provide other functionality suchas, for example, an acetoacetyl group).

An alcohol can also be reacted with PCl₃ or P₂O₅ to produce a phosphate.

The interior end groups within the cored dendrimer are capable offorming a coordinative bond or an ionic bond with a metal-containingprecursor such as a metal salt (i.e., the metal salt contains metalions). In some embodiments, the pH is adjusted to convert the interiorend group to an ionic form. For example, the pH can be raised to formthe corresponding ionic base of an acid end group and the pH can belowered to form the corresponding ionic acid of a base end group. Themetal-containing precursor can be subsequently reacted to form a metalparticle within the cored dendrimer. That is, the cored dendrimerfunctions as a template for the preparation of metal particles. Themetal particle forms within the cored dendrimer in the central interiorregion that is free of organic material.

In some embodiments, only the interior end groups formed after removalof the core organic material or a derivative thereof are capable offorming a bond with a metal-containing precursor. The cored dendrimertends to be free of other groups that can bond with the metal-containingprecursor.

The metal-containing precursor includes a chemical species or compoundthat can form a bond such as a coordinative bond or an ionic bond withthe interior end groups of the cored dendrimer. Mixtures ofmetal-containing precursors can be used to prepare alloys. After forminga bond with an interior end group within the cored dendrimer, themetal-containing precursor is treated with a reducing agent to form ametal particle within the central interior region of the coreddendrimer.

In some embodiments, the metal particles are noble metals such as, forexample, silver, gold, platinum, and palladium. Suitablemetal-containing precursors for these noble metals include, for example,salts or acids that can be dissolved in water or a polar solvent toprovide metal-containing species (e.g., metal-containing ions).Exemplary metal-containing precursors include HAuCl₄, H₂PtCl₄, AgNO₃,K₂PdCl₄, K₂PtCl₆, K₂PdCl₆, and the like. The soluble metal-containingspecies can form a coordinative bond or an ionic bond with an interiorend group within the central interior region of the cored dendrimer. Thebonded metal-containing species can be reduced to a metal by exposure toa reducing agent such as sodium borohydride, sodium cyanoborohydride,lithium triethylborohydride, lithium aluminum hydride,diisobutylaluminum hydride, lithium tri-t-butoxyaluminum hydride, sodiumbis(2-methoxyethoxy)aluminum hydride, molecular hydrogen, elementalsodium, or the like.

When the metal-containing precursor includes a soluble form of silversuch as silver ions, silver metal can be formed by exposure to a strongbase such as, for example, 1,2,2,6,6-pentamethyl piperidine,triethylamine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), or otherorganic amines.

Other metal particles that can form within the cored dendrimer include,but are not limited to, nickel, copper, zinc, lead, rhodium, cadmium,cobalt, mercury, ruthenium, iridium, or mixtures thereof. Such metalscan be prepared from a metal-containing precursor in the form of a saltor acid that is soluble in water or a polar solvent to form ametal-containing species (e.g., metal-containing ion). Exemplarymetal-containing precursors include, but are not limited to, nitratesalts (e.g., Ni(NO₃)₂, Cu(NO₃)₂, Zn(NO₃)₂, Pb(NO₃)₂, Cd(NO₃)₂, Hg(NO₃)₂,Co(NO₃)₂, and the like); chloride salts (e.g., RuCl₃, IrCl₃, RhCl₃, andthe like); acids (e.g., H₂IrCl₆ and the like); and metal complexes ororganometallic compounds (e.g., Co[bis(2-ethylhexylsulfosuccinate] andthe like). After bonding to the interior end group, the metal-containingspecies can be reduced by exposure to a reducing agent such as, forexample, sodium borohydride, sodium cyanoborohydride, lithiumtriethylborohydride, lithium aluminum hydride, diisobutylaluminumhydride, lithium tri-t-butoxyaluminum hydride, sodiumbis(2-methoxyethoxy)aluminum hydride, molecular hydrogen, or elementalsodium.

In other embodiments, a first metal-containing species is bonded to theinterior end groups of the cored dendrimer and reduced to form a firstmetal particle. Upon introduction of a second metal-containing precursorin the form of a second metal-containing species (e.g., secondmetal-containing ion), the first metal is oxidized to a first solublemetal-containing species and the second metal-containing species isreduced to yield a second metal particle. For example, Cu²⁺ ions can befirst coordinated to the interior end group and reduced to yieldparticulate Cu. Upon exposure of the copper particles to silver ions, anoxidation-reduction reaction occurs that result in the oxidation ofcopper metal to cupric ions and the reduction of silver ions to silvermetal. Thus, the copper particles are replaced with silver particleswithin the central interior regions of the cored dendrimers.

The reactions to bond the metal-containing precursor to the interiorgroup of the cored dendrimer and to reduce the bonded metal-containingspecies are typically conducted in water, a polar solvent, or a solventmixture such as a biphasic system. The reactions often occur at roomtemperature (e.g., 20 to 25° C.) or at a temperature up to about 100° C.

In some embodiments, the cored dendrimer can be removed after theformation of the metal particles. The cored dendrimer can be dissolved,for example, in an organic solvent that does not dissolve the metalparticles.

In a second aspect, a composite particle is provided that includes ametal particle within a central interior region of a cored dendrimer.The cored dendrimer has crosslinked dendrons surrounding the centralinterior region and the central interior region is free of organicmaterial. The metal particle has a size that is no greater than an outerdimension of the cored dendrimer.

The cored dendrimers of the composite particles are crosslinkedpolymeric spheres that are free of organic material in the centralregion of the sphere. The cored dendrimer is formed by removing aninterior region of a crosslinked dendrimer after cleavage of theattachment groups between the core organic material in the centralregion and the dendrons. The cored dendrimers contain n^(th) generationmaterial (i.e., n is an integer of 2 to 20) with at least some of thelower generations of material removed. For example, some coreddendrimers can have the zero generation removed, the zero and firstgeneration removed, or the zero, first, and second generations removed.

The cored dendrimer contains a plurality of crosslinks between thedendrons and optionally between the individual molecular chains within adendron. The crosslinking of the individual outer molecular chains tendsto produce a cored dendrimer that is robust. The crosslink density tendsto be low enough for passage of metal-containing precursor and otherreactants used to form the metal particles into the interior of thecored dendrimer.

The growth of the metal particles within the cored dendrimer is usuallylimited by the location of the crosslinks. The crosslinks tend to hinderthe further growth of the metal particles. The metal particles can havedimensions corresponding to the cored dendrimer where the crosslinks areat the outer periphery of the dendron molecular chains. That is, thesize of the cored dendrimer often defines the maximum size of the metalparticles. When the cored dendrimer has crosslinks at locations otherthan at the periphery of the dendron molecular chains (i.e., thecrosslinks are along the length of the branches within the dendron), thelocation of the crosslinks tends to define the maximum size of the metalparticles formed within the cored dendrimer. In some embodiments, themetal particles have a size that is larger than the central region ofthe cored dendrimer that is free of organic material.

In some embodiments, metallic particles may be prepared of a size thatis greater than the size of a single cored dendrimer. In thisembodiment, the inorganic particle may be associated with more than onecored dendrimer. That is, the metal particle can form a composite withmore than one cored dendrimer.

The mean average particle size is typically less than 100 nanometers. Insome embodiments, the mean average particle size is less than 80nanometers, less then 60 nanometers, less than 50 nanometers, less than40 nanometers, less than 30 nanometers, less than 20 nanometers, or lessthan 10 nanometers. The mean average size is usually no less than 0.1nanometers, no less than 0.2 nanometers, no less than 0.5 nanometers, noless than 1 nanometer, or no less than 2 nanometers. For example, themean average particle size can be in the range of 0.1 to 100 nanometers,in the range of 0.5 to 50 nanometers, in the range of 0.5 to 40nanometers, in the range of 0.5 to 30 nanometers, in the range of 0.5 to20 nanometers, or in the range of 0.5 to 10 nanometers.

In some embodiments, the composite particle is free of covalent bonds orionic bonds between an outer surface of the metal particle and the coreddendrimer. That is, the cored dendrimer does not have groups along thelength of the molecular chains of the dendrons or at the outer peripheryof the molecular chains of the dendrons that can form a coordinativebond or an ionic bond with a metal-containing precursor or metalparticle. The only groups capable of forming such a bond with ametal-containing precursor, metal particle, or both are the interior endgroups.

The lack of additional groups capable of bonding with a metal-containingprecursor, metal particle, or a combination thereof results in theformation of metal particles that have an outer surface that isnon-passivated and that is not bonded to the core dendrimer. This can beadvantageous for applications such as catalysis where coordinated metalsurfaces tend to be less reactive than metal surfaces that are notcoordinated.

In a third aspect, a composition is provided that includes an organicmatrix and a composite particle in the organic matrix. The compositeparticle contains a metal particle within a central interior region of acored dendrimer. The cored dendrimer has crosslinked dendronssurrounding the central interior region and the central interior regionis free of organic material. The metal particle has a size that is nogreater than an outer dimension of the cored dendrimer.

The organic matrix usually contains polymerizable monomers, polymericmaterial, or a combination thereof. The composite particles can be mixedwith previously formed polymeric material such as polyolefins,polyesters, polyurethanes, poly(meth)acrylates, polystyrenes,polycarbonates, polyimides, and the like. Alternatively, a polymericmaterial can be formed in the presence of the composite particles frompolymerizable monomers.

Representative examples of polymerizable monomers that can be used toform the organic matrix of the composite material include(meth)acrylates, (meth)acrylamides, olefins, styrenes, epoxides and thelike. Also, reactive oligomers such as acrylated or methacrylatedpolyesters, polyurethanes, or acrylics may also be used. The resultingcomposite material may be shaped or coated and then polymerized, forexample, via a free-radical mechanism. Photopolymerization may beinitiated by the use of a photoinitiator such as that commerciallyavailable from Ciba Specialty Chemicals, Tarrytown, N.Y. under the tradedesignation “IRGACURE” and “DAROCUR” or from BASF, Mt. Olive, N.Y. underthe trade designation “LUCERIN”. Other suitable polymerizationinitiators include, for example, organic peroxides, azo compounds, andthe like.

The crosslinked polymeric shell of the composite particle can functionto protect the particles from agglomeration when placed in an organicmatrix. Further, the crosslinked polymeric shell tends to dictate thesolubility properties of the composite particles as well as thecompatibility with various binder systems and solvents. For example,polymeric crosslinked shells that are non-polar, such as those formed bycrosslinking poly(benzyl ethers), tend to be compatible with organicmatrixes such as, for example, polystyrene or poly(methyl methacrylate).In another example, polymeric crosslinked shells that are aliphatic,such as those formed by crosslinking poly(amidoamines), tend to becompatible with more polar organic matrixes such as polymethacrylicacid, polyacrylic acid, polyacrylamide, polymethacrylamide, and thelike.

The composite particles or compositions that include the compositeparticles can be used for a variety of applications. For example,composite particles such as those that contain noble metals can be usedfor catalysis of various chemical reactions of commercial importance.Additionally, the composite particles can be added to an organic matrixto alter the refractive index of the organic matrix, increase the x-rayopacity of the organic matrix, or both. In some examples, the organicmatrix can remain optically transparent even though the refractive indexor x-ray opacity has been increased.

EXAMPLES

These examples are for illustrative purposes only and are not meant tobe limiting on the scope of the appended claims. All parts, percentages,ratios, etc. in the examples and the rest of the specification are byweight, unless noted otherwise. Solvents and other reagents used wereobtained from Sigma-Aldrich Chemical Company; Milwaukee, Wis. unlessotherwise noted.

Test Methods

TEM Analysis

An assessment of the particles sizes (and size distribution) for eachtype of material was performed using transmission electron microscopy(TEM). Once the initial survey images were obtained for each sampletype, more detailed images were prepared for particle counting andsizing.

The samples that were analyzed using TEM were typically dispersed in aliquid. To perform the TEM analysis, the samples were shaken well and 2microliters of the dispersion was placed onto a TEM grid using amicropipette. The TEM grids were ultrathin carbon/formvar microscopygrids that were obtained from Ted Pella, Inc., Redding, Calif. Theultrathin carbon/formvar grids were pre-treated by dipping the gridsconsecutively into acetone (5 seconds), chloroform (20 seconds), andacetone (5 seconds) to dissolve away the formvar region of the supportfilm.

The electron micrographs were obtained using either a Model H9000transmission electron microscope, operating at 300 kV, from Hitachi HighTechnologies America, Inc., Pleasanton, Calif., or a Model 200CXtransmission electron microscope, operating at 200 kV, from JEOL-USA,Inc., Peabody, Mass. Digital images were recorded using a GATAN model794 CCD camera. Bright field/dark field images were recorded forparticle sizing and high resolution electron microscopy images (HREM)were used for verification of particle crystallinity and particle shape.The HREM imaging mode allowed for atomic resolution of the latticespacing in the particles.

TEM bright field images were obtained from the transmitted beam whensome or all of the scattered (diffracted) electron beam was blanked offwith an objective aperture. Any features that caused the electron beamto scatter (such as a small particle, grain boundary or dislocation) ordiffract (grain oriented in a diffracting condition) generally appeareddark in a TEM bright field image.

TEM dark field images were formed from a specific diffracted beam fromthe diffraction pattern of the particles being viewed. Using thistechnique, only information from a specific diffraction condition (e.g.from a particular particle oriented for strong diffraction) appearedbright. Additionally, any features that were scattering into thespecific diffraction spot appeared bright (e.g. defects, grainboundaries, dislocations, stacking faults, or twins).

Preparative Example 1

A cored dendrimer containing eight interior carboxylic acid functionalgroups was prepared and characterized as detailed by S. C. Zimmerman etal., Nature, vol. 418, 399-403 (2002) and Wendland et al., J. Am. Chem.Soc., 121, 1389-1390 (1999). The core organic material was5,10,15,20-tetrakis(3,5-dihydroxylphenyl)-21H,23H-porphine. The dendronsattached to the core organic material had the following structure.

The attachment group was a carboxylate ester. The alkenyl crosslinkablegroups on the dendrons were crosslinked using a Grubbs' rutheniumalkylidene catalyst. The attachment group was cleaved using potassiumhydroxide in a tetrahydrofuran-ethanol-water mixture.

Example 1

A solution (0.002 moles/liter in chloroform) of the cored dendrimerprepared in Preparative Example 1 (100 microliters) was placed in a 2 mLglass vial. Silver nitrate (1.3 milligrams) and1,2,2,6,6-pentamethylpiperidine (PMP) (10 microliters) were added andthe resulting mixture was shaken for 2.5 hours to form a dark brownsolution. A UV/Vis spectrum (Hewlett Packard 8452 Diode ArraySpectrometer, Palo Alto, Calif.) of the solution in chloroform revealeda plasmon-absorption peak (λ_(max)=422 nm) consistent with particleformation. FIG. 1 is a TEM of Example 1.

The particle size distribution is shown in FIG. 2. The silver particleswithin the cored dendrimers had mean particle size of 3.5 nanometers.The particle size distribution was determined by making digitalmeasurements of individual particles from at least ten different TEMgrids. The total number of particles measured was between 100 and 300.

Example 2

Example 2 was a film that contained composite particles suspended inpolystyrene. The composite particles were those prepared in Example 1(silver particles within cored dendrimers). The polystyrene had a weightaverage molecular weight (M_(w)) of 280,000 g/mole.

Comparative Example 1 was a film that contained cored dendrimerssuspended in polystyrene (i.e., there was no metal particle within thecored dendrimer). The cored dendrimers were prepared in PreparatoryExample 1. The polystyrene had a weight average molecular weight (M_(w))of 280,000 g/mole.

Comparative Example 2 was a film that contained polystyrene (i.e., therewere no cored dendrimers and no composite particles). The polystyrenehad a weight average molecular weight (M_(w)) of 280,000 g/mole.

Table 1 gives the mass percentages of each component in the chloroformsolutions used for film coating.

In each case, films were cast via spin coating (750 rpm, 30 seconds)onto aluminum coated glass slides (100 nm thick Al on glass). The spincoater was manufactured by Headway Research, Inc., Garland, Tex. and theglass slides were microslides purchased from Corning Glass, Corning,N.Y. Variable angle spectral ellipsometry was used to determine the real(n) and imaginary (k) components of the refractive index at 421 nm foreach film. The ellipsometer was a M2000-U Spectral Ellipsometer from J.A. Wollam, Lincoln, Nebr.

TABLE 1 Refractive Index Cored Composite Sample Poly(styrene) dendrimerparticles (Ag) n k Example 2 0.44% 0.20% 1.518 0.0387 Comparative 0.44%0.20% 1.487 0.0231 Example 1 Comparative 0.44% 1.512 0.0168 Example 2

We claim:
 1. A composite particle comprising a cored dendrimercomprising crosslinked dendrons surrounding a central interior regionthat is free of organic material, wherein the cored dendrimer hasinterior end groups and crosslink locations, a metal particle comprisinggold, platinum, palladium, or mixtures thereof within the coreddendrimer, wherein the metal particle occupies the central interiorregion of the cored dendrimer and has a size that is determined by thecrosslink locations and wherein the interior end groups are the onlygroups of the cored dendrimer that can form a coordinative or ionic bondwith the metal particle.
 2. The composite particle of claim 1, whereinthe metal particle has a mean average particle size less than 50nanometers.
 3. The composite particle of claim 1, wherein the outersurface of the metal particle is not bonded to the cored dendrimer. 4.The composite particle of claim 1, wherein the cored dendrimer comprisesa crosslinked poly(benzyl ether).
 5. The composite particle of claim 1,wherein the metal particle comprises nickel, copper, zinc, lead,rhodium, cadmium, cobalt, mercury, ruthenium, iridium, or mixturesthereof
 6. The composite particle of claim 1, wherein the crosslinkeddendrons are crosslinked at an outer periphery of the cored dendrimer.7. The composite particle of claim 1, wherein the metal particle has anouter surface that is non-passivated.
 8. A composition comprisingcomposite particles and an organic matrix, the composite particlescomprising a cored dendrimer comprising crosslinked dendrons surroundinga central interior region that is free of organic material, wherein thecored dendrimer has interior end groups and crosslink locations; and ametal particle comprising gold, platinum, palladium, or mixtures thereofwithin the cored dendrimer, wherein the metal particle occupies thecentral interior region of the cored dendrimer and has a size that isdetermined by the crosslink locations and wherein the interior endgroups are the only groups of the cored dendrimer that can form acoordinative or ionic bond with the metal particle.
 9. The compositionof claim 8, wherein the metal particles have a size wherein the metalparticle has a mean average particle size less than 50 nanometers. 10.The composition of claim 8, wherein the cored dendrimer comprises acrosslinked poly(benzyl ether).
 11. The composition of claim 8, whereinthe organic matrix comprises a polymeric material, a polymerizablematerial, or a combination thereof.
 12. The composition particle ofclaim 8, wherein the outer surface of the metal particle is not bondedto the cored dendrimer.
 13. The composite particle of claim 8, whereinthe metal particle comprises nickel, copper, zinc, lead, rhodium,cadmium, cobalt, mercury, ruthenium, iridium, or mixtures thereof 14.The composite particle of claim 8, wherein the crosslinked dendrons arecrosslinked at an outer periphery of the cored dendrimer.
 15. Thecomposite particle of claim 8, wherein the metal particle has an outersurface that is non-passivated.